Invoice improvements

We would like to be able to define custom fields on invoices - we like to track various attributes of invoices, and would like to add fields for those attributes to invoices, then allow us to set/modify the values in those fields, and allow us to include those fields in reports related to invoices.

We would like to create reports based on invoices, such as all invoices by client/project and total hours / billings on each invoice, as well as the custom fields described above.

The Invoice List page includes a column named "External System Invoice #", yet that field is not editable in any way. We currently do not use the integration with our accounting system, yet would like to be able to capture our invoice numbers. Can this field be added to the forms where you add and edit an invoice?